Traction drive transmissions are well known and have been used for years in applications where efficiency, and the power to weight ratio, were not of primary concern. Up to the present time, due to their multiple power paths, the most successful of the commercially available traction drives has been the planetary type or the multidisc type. However, even these are high weight to power units. High power density traction drives have proved to be far more difficult to perfect, and there are very few, if any, examples in use today. The inherent advantages of using rollers rather than gears to transmit power, such as smooth, vibration free power transfer, are offset by equally significant design problems. Foremost of these problems is the very low coefficient of traction. This severely limits the tangential force that can be transmitted through the traction engagement area. In a conventional traction drive, the amount of tangential force required is a function of the output torque. This is why one rarely sees a conventional traction drive with an input/output speed ratio above 7:1.
In order to transmit any appreciable power through the traction engagement area, an extremely high force, normal to the traction engagement area, must be used. This force imposes considerable load on the support bearings and support structure. This is the principal reason that conventional traction drives have such a low power density.
To reduce Hertzjan stress, the traction engagement area is enlarged. However, the geometry of existing traction drives allows very little flexibility in the size or shape of its traction engagement area. The length of the engagement area (in the rolling direction) is predetermined by the rolling radius of the mating elements. The only parameter which can be changed is the width. Thus, the majority of traction drives in use today, have engagement area ellipses in which the average width of the engagement area (transverse to the rolling direction) can be four times the length. Although rolling motion is intrinsically very efficient, with losses approaching zero, power losses caused by the lubricant can be significant. The two primary causes are; shearing of the lubricant and pressure transients of the lubricant, in the engagement area. In general, the wider the engagement area, the higher the power losses. These power losses show up in the form of heat. U.S. Pat. Nos. 2,020,667, 1935; U.S. Pat. 3,099,927, 1963; U.S. Pat. 3,318,164, 1967; and U.S. Pat. 4,369,667, 1983, demonstrate the persistence of this problem. The fact that many conventional traction drives use excessive force, normal to the engagement area, is further evidence of the efforts being made to increase the capacity of these traction drives without further increasing the width of the engagement area.
While these disadvantages are overcome in U.S. Pat. No. 5,051,106, 1991, it should be observed that the axes of some of the shafts in this transmission are transverse. Whereas, all the shafts of the present invention are parallel, thus simplifying the machining.
For the most part, the gains made in traction drive technology in the past have come about from improvements made in the fields of metallurgy and lubrication, and not from major breakthroughs in traction drive design.